This invention relates to inverters of the type comprising two sub-inverters in general and more particularly to an improved method and apparatus for controlling such an inverter so as to minimize the size of choke needed.
It has been recognized that by connecting in parallel two identical inverters, hereinafter referred to as sub-inverters, an inverter having an advantageous control characteristic can be constructed. Typically, the sub-inverters will be provided with thyristors or switching transistors used as controlled rectifiers. The advantage of a self-commutating inverter of a design of this nature is that on one hand the output power is increased and on the other hand, the output voltage can be controlled in fine steps by assigning an individual switching pattern to each sub-inverter.
The typical converter of this type comprises two three-phase sub-inverters having inputs connected to a DC voltage source. Typical of this type inverter is that described in the conference report "Conference Report, IEEE International Semiconductor Power Converter Conference," Baltimore, Md., USA, May 8 to 10, 1972, page 2-4-1 to 2-4-6., particularly FIG. 3. The respective output terminals of the sub-inverters are connected through a choke which has a center tap. The three-phase voltage output of the inverter is then taken off at the three center taps. The two AC output voltages provided as inputs to the choke are displaced with respect to each other by an adjustable control angle. The chokes insure that the AC output voltages are averaged. In this prior art inverter, the resulting voltage and thus, the voltage at the load is controlled by changing the control angle. A control unit is used to displace the control pulses which are provided to the sub-inverters with respect to each other such that the desired control angle and as a result, the desired voltage at the load is obtained. This method is referred to as the slewing method, addition method or the rotary transformer principle.
From the dissertation of H. Stemmler "Control Methods for Single- and Multi-pulse Subharmonic Inverters for the supply of Squirrel-Cage Rotor Motors," Rheinish-Westphalian Technical University Aachen, 1970, FIG. 5/III.4 on page 363 and the accompanying description on page 81, the three-phase inverter described above is known but without the above mentioned slewing method. As described, a total of six single phase sub-inverters supplied from a common DC voltage source are provided. These are combined in pairs to form single-phase inverters with the two sub-inverters of a pair connected together through a suction coil with a center tap. The center taps of the three coils or chokes are connected to a Y-connected three phase rotating field machine. The magnetic design of each of the three chokes must be such that it can withstand the difference voltage time area of the AC output voltages of the two sub-inverters. Thus, each choke maintains a share of the DC voltage being supplied for certain time intervals. This requires that the choke be of considerable size in turn resulting in a large weight of the three inverters. This is a severe disadvantage, particularly where the volume and/or weight of the inverter must be kept as small as possible. An example of such an application is where the inverter is to be used for the propulsion of a self-propelled electric vehicle. In such a case, the smallest possible chokes must be used to save weight and at the same time, reduce cost.
Thus, it can be seen that there is a need for an inverter that offers the advantages of the slewing method as noted above but at the same time can be constructed using a smaller choke than would otherwise be necessary.